Bicarbonate transport in collecting tubules from outer stripe of outer medulla of rabbit kidneys

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Abstract
The purpose of this study is to characterize the features of bicarbonate (total CO2) transport in isolated perfused collecting tubules obtained from the outer stripe of the outer medulla (OMCTos) of rabbit kidneys. Under control conditions (25 mM HCO3- in the perfusate and bath), all OMCTos studied absorbed total CO2 at a mean rate of 8.61 +/- 0.44 pmol.mm-1.min-1. Ouabain (10(-4) M in the bath) did not affect the rate of total CO2 absorption (JtCO2). Addition of the diethylstilbene 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) in a concentration of 10(-4) M or replacement of bath chloride by gluconate reduced JtCO2 by approximately 50%, whereas replacement of luminal chloride increased JtCO2 by 40%. The carbonic anhydrase inhibitors acetazolamide and ethoxyzolamide in concentrations of 10(-4) M had little effect on JtCO2. In a concentration of 10(-3) M, acetazolamide reduced JtCO2 by only 31%. OMCTos obtained from rabbits with ammonium chloride-induced metabolic acidosis did not have increased rates of total CO2 absorption compared with the control, but treatment of animals with mineralocorticoids increased JtCO2. These results indicate that OMCTos are capable of significant bicarbonate absorption in vitro. This absorption 1) is independent of sodium transport, 2) appears to require, at least in large part, HCO3- or OH- -Cl- exchange across the basolateral cell membrane of acid-secreting cells, 3) is much more resistant to inhibition by carbonic anhydrase inhibitors than reported previously for other rabbit nephron segments, and 4) is stimulated by prior mineralocorticoid treatment of animals but not by prior metabolic acidosis in vivo.